This application relates to long persistence red emitting phosphors and their preparation. More particularly, this invention relates to strontium sulfide based phosphors activated with a divalent rare earth and a trivalent rare earth that does not change valence in the strontium sulfide host.
Long persistence phosphors that emit in the green, blue-green and blue colorations have been known for some time. They include zinc sulfide-based and strontium aluminate-based phosphors activated with rare earths.
However, orange and red hued long persistence phosphors have been disclosed only recently, and they have several serious drawbacks in use. For example, Royce et al discloses in U.S. Pat. No. 5,650,094 rare earth activated divalent titanate phosphors, such as CaTiO3 and Caxe2x80x94Znxe2x80x94Mgxe2x80x94TiO3, but their emission is visible only for a few minutes. Lindmayer, in U.S. Pat. No. 5,043,096, reported a strontium sulfide based phosphor doped with three rare earths in the form of their oxides and fluxed with a halide, such as LiF. However, the fired fluoride phosphor was highly sintered and had to be ground to obtain a useful powdered material. However, grinding degrades the emission, and thus the phosphor must be heated or annealed to repair the crystal defect damage. However, the emission performance is never fully restored. These phosphors are described as being useful as additives to paint formulations.
The above phosphors are different from stimulable phosphors. Stimulable phosphors, when exposed to ultraviolet or visible light, only weakly or poorly luminesce, and when the light source is removed, the luminescence ceases. However, when the phosphor is later exposed to infrared light, which can be hours or months later, the phosphor emits a strong burst of light. Thus these phosphors store the original light energy and thus are known as storage-type phosphors. Such phosphors are disclosed by Keller for example in U.S. Pat. No. 2,979,467, by Kabay et al in U.S. Pat. No. 4,857,228 and by Lindmayer in U.S. Pat. No. 4,705,952. These phosphors are characterized by being doped by a rare earth that changes its valence, such as samarium or bismuth, among others. This behavior is quite different from the long persistence phosphors described herein which, after exposure to light, continue to luminesce after the light source has been removed. Such phosphors do not exhibit storage effects.
Long persistence phosphors that emit in the orange to red are highly desirable because they are easy to see in the dark, particularly in the event of a power failure for example. They are highly useful for xe2x80x9cEXITxe2x80x9d signs in a darkened theater, to mark fire fighting equipment, to make visible other safety devices and breathing masks, fire axes and the like. Safety devices and signs such as the above have long used red colors, and thus they are familiar. In addition, for decorative or novelty uses, such as for toys, automobile hub caps, sporting goods and the like, red and orange are also very desirable bright colors.
Thus a red-orange-emitting, long persistence phosphor has long been sought.
We have found that strontium sulfide (SrS) activated with divalent europium and co-activated with a trivalent rare earth that does not change valence in the SrS host, and including a halide and oxygen, forms long lasting, red and orange phosphors after firing. The present solid, sintered phosphors can be molded directly into a desired shape during firing, or, if mixed with alumina powder, can form a phosphor powder directly. Alternatively, a SrS:Eu phosphor doped with a trivalent rare earth ion that does not change valence in the SrS host, can be fired in sulfur vapor contacted to or passed over a carbon source to produce a CS2 atmosphere. This method also forms a phosphor powder directly.